In the field of implantable medical devices, polymers based on L-lactic acid are quite favored due to their excellent biocompatibility. That is, lactic acid polymers biodegrade to smaller fragments and eventually to small molecules including lactic acid itself, a naturally occurring compound in the mammalian system. Thus, the degradation products of polylactic acids tend to be generally well-tolerated by subjects.
When synthesizing poly(lactic acid) it is possible to use lactic acid itself as the monomer. The molecular weight of the polymer obtained, however is limited and if higher molecular weight product is desired, lactide, the dimer of lactic acid is the preferred monomer. For the purposes of this disclosure, poly(lactide) will be used to signify that the resultant polymers may have a broad range of molecular weights.
Constructs made of poly(lactide) exhibit good mechanical characteristics such as strength and tensile modulus. The fracture toughness of poly(lactide) is, however, lower than is often desired in a particular construct, for example in implantable medical devices such as stents. The high strength and tensile modulus and concomitant low fracture toughness stems from the high degree of crystallinity of poly(lactide), which is about 37%. In addition to the relatively high percent crystallinity, the crystalline structure of poly(lactic acid) in general comprises relatively large spherulites that add to the strength and tensile modulus of the polymer but detract from the fracture toughness.
What is needed is a lactide-based composition that can be fabricated into an implantable medical device that exhibits strength, a good tensile modulus and improved fracture toughness. The present invention provides such a composition and implantable medical devices fabricated of that composition.